Cells of most vertebrate species are anchorage dependent and undergo mitosis and proliferate only when firmly attached to a surface. Such cells often secrete their own adhesion proteins (e.g., fibronectin (FN)), which facilitate their adhesion to extracellular substrate surfaces or to other appropriate cells. The mechanism of cell adhesion is little understood. Recently, synthetic peptides derived from the sequence of FN have been used for competitive inhibition of its functions in vitro. We have started to examine this system for the possible effects of homologous and heterologous peptide associations upon peptide function and receptor binding. The degree of peptide aggregation could be measured by sedimentation equilibrium experiments in the analytical ultracentrifuge. We determined that sedimentation equilibrium was achieved in an overnight centrifugation run in phosphate buffered saline, pH 7.2. Such studies were carried out on these peptides and the percent dimer determined for each peptide at a series of peptide concentrations. For such small peptides, the effects of changes in amino acid composition or sequence on peptide properties and percent dimer were significant and easily seen. We hope the results of these studies will help to better understand the mechanism of cell adhesion.